首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   5297篇
  免费   357篇
  2023年   18篇
  2022年   55篇
  2021年   98篇
  2020年   45篇
  2019年   81篇
  2018年   117篇
  2017年   95篇
  2016年   152篇
  2015年   219篇
  2014年   274篇
  2013年   410篇
  2012年   453篇
  2011年   436篇
  2010年   275篇
  2009年   206篇
  2008年   343篇
  2007年   344篇
  2006年   321篇
  2005年   321篇
  2004年   281篇
  2003年   229篇
  2002年   245篇
  2001年   53篇
  2000年   47篇
  1999年   56篇
  1998年   50篇
  1997年   37篇
  1996年   43篇
  1995年   39篇
  1994年   30篇
  1993年   41篇
  1992年   28篇
  1991年   15篇
  1990年   17篇
  1989年   16篇
  1988年   15篇
  1987年   16篇
  1986年   14篇
  1985年   12篇
  1984年   21篇
  1983年   14篇
  1982年   6篇
  1981年   15篇
  1980年   9篇
  1979年   7篇
  1978年   6篇
  1977年   6篇
  1975年   3篇
  1973年   3篇
  1965年   3篇
排序方式: 共有5654条查询结果,搜索用时 15 毫秒
91.
Summary

Observations on growth and development of Salvia longistyla Benth., in several photoperiodic and termoperiodic conditions, both natural and artificial, were performed. The conclusions of my work are the following

I° - It has been proved that Salvia longistyla is a typical short-day plant and that its critical point of reaction lies approssimately on the 14 hours, when the average night-temperature is about 18°C; the flowering does not succeed if the night-temperature is decidedly below 15°C.

II° - The plant' requires short-day conditions not only for the formation of the flower buds, but also for the subsequent development, i. e. the full bloom; consequently the plant belongs to Eguchi's S. S. class (short day-plant for both the manifestations).

III. - Under natural conditions, in our latitude, Salvia longistyla flowers only in Autumn, because only in this season we have suitable photoperiod and concomitantly suitable night-temperature. In Spring, though the day is short, the night-temperature is not sufficiently high; in Winter and in Spring, the flowering is possible only in a warm green-house, with a night-temperature of about 18°C. In orange green-houses Salvia longistyla can continue the flowering begun in Autumn; during Winter and Spring new blooms do not succeed, just because the night-temperature, in such green houses, is never sufficiently high.

IV. - We can obtain fruiting only by artificial pollination (lack of natural pollination agency).

V. - The basal branches do not flower even in optimal photo- and termoperiodical conditions (which allow apical flowering) on acount of a mechanism of correlative inhibition.

VI. - The elongation of the internodes is relatively inhibited at a very long photoperiod, differently from what we observe in most plants.

These prerogatives, verified in repeated experiments, are in perfect accordance with the characteristics of the ambient in which the plant is naturally diffused (Mexico). The altitude and latitude of the stations in which the plant has usually been found, have a short photoperiod all year long, and temperatures (also night-temperatures) costantly suitable for a good growth and bloom. The plants, in the natural ambient, are perennial and bushy. The pluri-annuality can be referred to the flowering inhibition of the lateral and basal branches, that takes place in a specimen and that keeps on for a long time, also after the full flowering of the principal axis. The basal shoots show a tendency to the vegetative growth, even though the external conditions are suitable to the flowering of the apical parts of the plant. Therefore we must attribute the pluriannuality of S. longistyla to this correlative flower-inhibiting mechanism.  相似文献   
92.
93.
IntroductionN-palmitoylethanolamine (PEA) is an endogenous fatty acid amide belonging to the family of the N-acylethanolamines (NAEs). Recently, several studies demonstrated that PEA is an important analgesic, antiinflammatory, and neuroprotective mediator. The aim of this study was to investigate the effect of co-ultramicronized PEA + luteolin formulation on the modulation of the inflammatory response in mice subjected to collagen-induced arthritis (CIA).MethodsCIA was induced by an intradermally injection of 100 μl of the emulsion (containing 100 μg of bovine type II collagen (CII)) and complete Freund adjuvant (CFA) at the base of the tail. On day 21, a second injection of CII in CFA was administered. Mice subjected to CIA were administered PEA (10 mg/kg 10% ethanol, intraperitoneally (i.p.)) or co-ultramicronized PEA + luteolin (1 mg/kg, i.p.) every 24 hours, starting from day 25 to 35.ResultsMice developed erosive hind-paw arthritis when immunized with CII in CFA. Macroscopic clinical evidence of CIA first appeared as periarticular erythema and edema in the hindpaws. The incidence of CIA was 100% by day 28 in the CII-challenged mice, and the severity of CIA progressed over a 35-day period with a resorption of bone. The histopathology of CIA included erosion of the cartilage at the joint. Treatment with PEA or PEA + luteolin ameliorated the clinical signs at days 26 to 35 and improved histologic status in the joint and paw. The degree of oxidative and nitrosative damage was significantly reduced in PEA + luteolin-treated mice, as indicated by nitrotyrosine and malondialdehyde (MDA) levels. Plasma levels of the proinflammatory cytokines and chemokines were significantly reduced by PEA + luteolin treatment.ConclusionsWe demonstrated that PEA co-ultramicronized with luteolin exerts an antiinflammatory effect during chronic inflammation and ameliorates CIA.  相似文献   
94.

Introduction

The diagnostic, predictive and prognostic role of anti-cyclic citrullinated peptide (CCP) antibodies in rheumatoid arthritis (RA) patients is widely accepted. Moreover, detection of these antibodies in subjects presenting with undifferentiated arthritis (UA) is associated with a significant risk to develop the disease. On the other hand, clinical and prognostic significance of evaluating anti-CCP levels in subjects with inflammatory arthritis at disease onset has not been fully clarified. The goal of this prospective study is to analyze the value and prognostic significance of anti-CCP titer quantification in UA subjects.

Methods

Serial anti-CCP assays were measured in 192 consecutive patients presenting with UA lasting less than 12 weeks. Clinical and serological data and arthritis outcome were evaluated every 6 months until two years of follow-up.

Results

Anti-CCP positivity, at both low and high titer, and arthritis of hand joints significantly predicted RA at two years, risk increasing in subjects with high anti-CCP titers at baseline. Moreover, time to RA diagnosis was shorter in patients with high anti-CCP2 titers at enrollment with respect to those with low antibody concentration.

Conclusions

Presence of anti-CCP antibodies, at both low and high concentration, is significantly associated with RA development in subjects with recent onset UA. However, time interval from the onset of the first symptoms to the fulfilment of the classification criteria appears to be directly related to the initial anti-CCP level.  相似文献   
95.
Abstract

The oxidation of purine derivatives using porphyrins as catalysts and dimethyldioxirane (DMDO) as oxygen atom donor is reported. The regioselectivity of the oxidation was found to be dependent on the presence of a free OH moiety on the N(9)-side chain of the substrate and on the structure of the catalyst.  相似文献   
96.
97.
In this contribution the results of a zoogeographical analysis, carried out on the 123 endemic leaf beetle species (Coleoptera: Chrysomelidae) occurring in Italy and its immediately adjacent regions, are reported. To assess the level of faunistic similarity among the different geographic regions studied, a cluster analysis was performed, based on the endemic component. This was done by calculating the Baroni Urbani & Buser’s similarity index (BUB). Finally, a parsimony analysis of endemicity (PAE) was used to identify the most important areas of endemism in Italy.  相似文献   
98.
99.
Removal of invasive species often benefits biological diversity allowing ecosystems’ recovery. However, it is important to assess the functional roles that invaders may have established in their new areas to avoid unexpected results from species elimination. Invasive animal-pollinated plants may affect the plant–pollination interactions by changing pollinator availability and/or behaviour in the community. Thus, removal of an invasive plant may have important effects on pollinator community that may then be reflected positive or negatively on the reproductive success of native plants. The objective of this study was to assess the effect of removing Oxalis pes-caprae, an invasive weed widely spread in the Mediterranean basin, on plant–pollinator interactions and on the reproductive success of co-flowering native plants. For this, a disturbed area in central Portugal, where this species is highly abundant, was selected. Visitation rates, natural pollen loads, pollen tube growth and natural fruit set of native plants were compared in the presence of O. pes-caprae and after manual removal of their flowers. Our results showed a highly resilient pollination network but also revealed some facilitative effects of O. pes-caprae on the reproductive success of co-flowering native plants. Reproductive success of the native plants seems to depend not only on the number and diversity of floral visitors, but also on their efficiency as pollinators. The information provided on the effects of invasive species on the sexual reproductive success of natives is essential for adequate management of invaded areas.  相似文献   
100.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号